String gps_xor_checksum(char *string)
{
	size_t i;
	uint8_t XOR;
	uint8_t c;
 
	XOR = 0;
 
	// Calculate checksum ignoring the first two $s
	for (i = 2; i < strlen(string); i++)
	{
		c = string[i];
		XOR ^= c;
	}
 
        String checksum = String(XOR,HEX);
        checksum[0] = toupper(checksum[0]);
        checksum[1] = toupper(checksum[1]);
        
        return checksum;
}
